AC myths 3: Not AC and fan together

Thursday 15 September 2011, 1.00pm HKT

In this third part, we continue our AC myth-busting by looking at why using the AC and fan together is all wrong, despite the strong temptation to do so.

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Bullshit #6
If 25°C (77°F) is too warm for you, run an electric fan on high with the AC set no cooler than 25°C. This will provide optimal efficiency to augment AC cooling.

Rubbish! If you are ever concerned about maintaining a low carbon footprint (and who isn’t?), this is exactly the way to skyrocket your carbon footprint — and your electricity bill.

Running your fan like that is crazy. You can factualise all day long, calculationalise endlessly, citationise everything, statisticise all the time — but you’ll be no wiser about your expenses. Until the bills arrive.

FACT: The AC works at its rated power consumption level regardless of the thermostat setting.

Not strictly accurate in the absolute engineering sense, but true in a relative sense from a consumer point of view.

The garden-variety electric or ceiling fan uses around 60 watts an hour (just like a 60-watt lightbulb). The average split-type (wall) AC uses 650 watts an hour. That’s like 10 times the power consumption. Some people balk at this and use that ‘fact’ to talk trash.

Sure it’s 10 times greater. And it has to be.

THINK. The AC has to do several things — sucking in air, compressing, decompressing, cycling the refrigerant, cooling the air, running the fan motor, dehumidifying and other stuff. All the fan has to do is spin the propeller motor. Whaddaya expect, professah?

Be objective. Running a fan in addition to the AC is just adding on to your overall power usage whilst not making the ambient temperature appreciably cooler or more comfortable. As said already, modern ACs run at their manufactured power consumption level regardless of the temperature being set to.

The fact that a 650-watt AC uses 10 times the electricity of a 60-watt fan (and therefore higher running cost) is irrelevant. Running a high power-consuming appliance has to be more expensive than running a cheapo — that much should be obvious. It’s the number of hours you put the AC or fan to use that also determines the price of your electricity bills, dimwit.

If you want more, you use more, so you pay more. The AC is like your bitchy girlfriend: you like her cleavage, but you have to put up with her drama. Your mother never told you this?

FACT: ACs are designed to create their own air drafts for balanced cooling and dehumidification. Therefore, running a fan will throw the AC airflow out of whack.

Disrupting the AC airflow causes the AC to work that much harder. In effect, you’re driving your AC to early death.

It’s like the bathroom ventilation fan. What goes out, something must come in. Whenever you turn on the bathroom fan, you need to throw open at least one window for air inflow to balance out the air outflow. Since the air outflow is ‘assisted’ (by the fan), the air inflow must be of a greater margin than the outflow. Still with me?

Shut off all the windows or doors, and the bathroom fan causes a pressure difference between indoors and outdoors. Then, the fan labours against the lack of airflow (sometimes audibly so), and burns out sooner than usual.

But why run another ‘cooling’ appliance (especially a fan) in conjunction with the AC? It makes no sense because of the above. Read some school-level physics, please.

FACT: The fan motor generates heat, and is counterproductive to the work of the AC.

You never thought of this, did you, genius?

For some people, a 25°C/77°F AC is sometimes unneeded altogether:—

“I find 25°C is a very comfortable temperature with [just] the fan on. And I grew up in Calgary [Canada] where 25°C is considered a hot summer day!” (Friend)

Use the AC as it was intended and it will do its job properly without using fans.

Trivia: American engineer Schuyler Skaats Wheeler (1860-1923) invented the electric fan in 1882 at age 22. A few years later, German-American inventor Philip Diehl (1847-1913) invented the electric ceiling fan by sticking blades to a sewing machine motor and attached it to the ceiling.

17 Responses to “AC myths 3: Not AC and fan together”

Your information is valuable, but did you need to be rude to make your point. You must be one of those arrogant “know it all” types. And that may not be true either because you haven’t learned respect. Thanks for the info but I could do with out the condescension.

Thank you for your interest, but, no, I wasn’t rude and wasn’t being rude. There’s nothing in this and the other instalments that is condescending. No, if you don’t mind, I’m not the know-it-all type. Would you mind very much just to tell ME how I haven’t learnt respect? You didn’t just latch on to the “Bullshit” subheadings, did you? Thanks for your comment, but I too could do without your pugnacious condescension.

I didn’t ask you to read this article and its other instalments. It isn’t like this article had been a direct reply to you. It isn’t as if we were in a thread of comments and I made a rude reply to you somewhere along the thread. If my article displeases you, then I’m sorry, but you can just step back from your cheese and crackers and not read it. YOU DIDN’T HAVE TO PUT IN DEROGATORY INSINUATIONS ABOUT ME BEING ARROGANT, CONDESCENDING AND NOT LEARNT RESPECT. You need to watch your step with others, Mr Stevens.

David is correct; you are being condescending. Quote: “You never thought of this, did you, genius?” This is almost the definition of condescension. And I’ve read the other parts of this post, which are even worse.

But what is worse, is that you are completely wrong. Turning on a ceiling fan will not cause your AC to work harder, as all it is doing is moving air around the room. A window fan may have a marginal effect for the reasons you mention, but no one is suggesting that you run a window fan while the AC is on.

Yes, a fan heats up, but the effect of this is negligible to the cooling effect of moving air. Your body cools itself better when air is moving over your skin. This allows you to feel cooler even in a room with a higher ambien temperature. You save energy by setting your thermostat to a higher temperature while remaining comfortable (and I’ve read your part 5: it is also incorrect).

So please don’t talk down to us. You may not have asked us to read this article, but you did publish it on the internet for everyone to read, so don’t throw a fit when someone calls you out for being a jerk.

I’ve just been on the blower with my HVAC technician, which I need not have to, but I also wanted to have a second opinion.

Straight from the horse’s mouth: Using a ceiling fan WILL cause the AC (especially a split-type AC) to work harder because it DISRUPTS THE AC AIRFLOW for optimal operation. (This is exactly what I’ve said in 12th paragraph, the one just after the second fact.)

How much harder? Technician said that depends largely on the size of the premises. The larger the premises, the less stress on the AC. Conversely, the smaller the premises, the more stress.

No, I’ve not been completely wrong. But you’ve highlighted in a very good way an area in this topic that has caused many arguments.

That totally nonsense here. You have no clue about anything and only mixed up certain things your heard as myth from others! To rise the indoor temperature from 20 deg. C to 25 deg. C in general has only one reason: Saving energy and lower CO2 emission by lowering AC compressor running time! And as a matter of fact it will be lower on similar outdoor temp. conditions, respectively delta T. However how intelligent that you advise cooling more down the indoor but wearing more if feeling uncomfortable… Never heard such bullshit… And regarding which international standard is the normal temperature 20 deg. C again???

Do not agree to fan not being used with AC. Cut all the crap and do it practically in a room of 15ft by 10ft with a 1.5 TR AC. Measure the temperatures at various quadrants of the room, after 10 min. Of running the AC without and with the fan. See for yourselves the difference before quoting principles of Physics in such shallow understanding. The room will have homogeneous cooling when fan is on. The AC will use lessor power with compressor taking longer rests. Besides, warm air being lighter, moves up and clouds the ceiling area and eventually needs to be cooled to get effective room cooling. The sooner the warm air is circulated and passed through the closed circuit cooling system of the AC, the sooner will the AC compressor get into the long switch-off and short switch-on regime, thereby saving power.

In other words, switch on the AC for 10 minutes, switch on the fan, and measure the temperatures at various points of the room. Then remeasure with the fan switched off, and compare the results. Right?

The fan would distribute cooled air more evenly than otherwise, right?

The more even mixing of cooled and uncooled air will have homogenised the overall air, thereby leading the AC compressor to run shorter cooling cycles, right?

Warm air is less dense than cool/cooled air, so the sooner the warm air gets cooled via the AC, the sooner the AC gets to operate on non-cooling mode, right?

Yes, I get that. We ALL get that. It doesn’t work out that way in real life. It varies geographically and situationally.